A number of testing systems are relied upon to determine the physical condition of an individual. These systems include measuring physiological parameters in the individual such as their anaerobic or lactic acid threshold, maximal oxygen uptake (VO2 max), maximal heart rate, heart rate variability (i.e. beat-to-beat alterations in heart rate), heart rate recovery, and muscle sympathetic nerve activity (MSNA).
For example, it is known that exercise training increases heart muscle function so that for each beat, the heart's capability to deliver blood to the tissues, including skeletal muscles, is increased and, when combined with training induced increases in the ability of skeletal muscles to extract and utilise oxygen from the blood, results in an increase in VO2 max. Exercise training also alters the mobilisation and usage of fuel molecules by skeletal muscle such that more fat is oxidised with less reliance on carbohydrate as a fuel, which reduces blood lactic acid concentrations and increases the lactic acid or anaerobic threshold. Therefore, it is generally accepted that the better condition an individual is in, the higher will be their VO2 max and lactate threshold, and the lower their heart rate with the same exercise workload. This lower heart rate at the same exercise workload is attributable to an increased stroke volume, and thus requires reduced activity of the sympathetic division of the autonomic nervous system. Thus, a person in better physical condition will have a lower heart rate at any given exercise workload, reflecting reduced sympathetic nervous system activity.
Furthermore, decrements in physical performance have been associated with changes in beat-to-beat variation in heart rate and heart rate recovery, which reflect alterations in the balance of activity between the sympathetic and parasympathetic divisions of the autonomic nervous system, and MSNA, which reflects the level of activity of sympathetic neurons supplying skeletal muscle.
Alterations in the balance of activity between the sympathetic and parasympathetic divisions of the autonomic nervous system have also been identified in the pathogenesis of a number of cardiovascular diseases, including coronary artery disease, hypertension, congestive heart failure, arrhythmias and sudden cardiac death. The magnitude of the autonomic imbalance correlates with the severity of the disease, and favours an increasing predominance of sympathetic activity and reduced parasympathetic activity. Small changes in autonomic balance that precede the presence of overt disease could therefore be predictive of future cardiovascular disease risk. Accordingly, measures which are able to sensitively detect changes in autonomic balance that precede the presence of overt disease will be useful for predicting future cardiovascular disease risk and/or tracking disease progression.
Measurement of physiological parameters related to autonomic balance presents its problems. For example, the “gold standard” method of measuring variations in beat-to-beat heart rate responses to assess heart rate variability requires an individual to lie quietly for up to twenty minutes while an electrocardiogram is recorded, and then requires specific expertise to analyse the recording.
Measurement of physiological parameters related to exercise performance also presents challenges. VO2 max and anaerobic threshold tests often rely on an individual performing progressive physical exercise over time to a maximum effort, with analysis of gas concentrations in expired air and blood sampling occurring periodically during the exercise to measure for oxygen uptake and the levels of lactic acid in the individual's blood. Other tests such as the Astrand-Rhyming test involve the individual exercising at a sub-maximal capacity for a period of time to achieve a steady-state heart rate and then using a nomogram to predict their maximal oxygen uptake from the relationship between heart rate and exercise workload, or the Conconi test in which an individual's running speed is plotted against their heart rate for predetermined distance intervals, again both require maximal exercise. Still further, tests which determine MSNA require measurements of nerve activity in the peroneal nerve using the microneurography technique which includes the use of invasive procedures and sophisticated machinery not typically available to all.
Accordingly, the aforementioned tests are either overly invasive, expensive or require specific high-level expertise to conduct (and are therefore impractical for a majority of individuals), are inaccurate, or they only provide meaningful results if the individual being tested can perform an aerobic activity to their maximal capacity, which is not always possible or recommended, particularly in individuals who may have some underlying medical condition.
Furthermore, it is often necessary to measure multiple parameters to achieve a complete overall idea of the physiological status of an individual, as single measures, such as VO2 max, when taken alone, are not always useful for predicting athletic performance. For example, two individuals may have the same VO2 max but one may consistently perform better than the other in athletic competition because the lactic acid threshold of the former individual may be higher than that of the latter. Thus, there is at present no single testing method which can provide a complete indication of the physiological readiness to perform an athletic activity, and thereby predict athletic performance, other than to have an individual undertake the performance, which is not always possible or practical.
Athletic training to improve sporting performance is based on the principle that progressively overloading physiological systems, with adequate recovery time for adaptation to take place, results in improved performance. However, if insufficient recovery occurs after a bout of exercise before an additional load is undertaken, then this can lead to over-reaching, with a temporary reduction in performance. If additional overload is then maintained this may lead to a state of over-training with long-term performance deficits and adverse impacts on the health of the individual.
There are a number of physiological, biochemical, psychological and immunological signs and symptoms that have been associated with over-reaching or over-training. However, tests that can identify markers which precede the ultimate drop in performance associated with these signs and symptoms have not been available. Unfortunately, due to the inherent limitations of the tests described above, they do not allow the physical status of an individual to be determined in terms of their readiness to perform exercise in an accurate, meaningful, and/or practical manner.
Similarly, there is currently no objective method or test for determining the recovery state of an individual, such as an athlete, and/or to predict the athlete's ability to perform an athletic activity after a period of recovery that is widely accepted other than to get them to undergo a performance test and compare the level of performance achieved with a measure that was taken when the athlete was known to be fully recovered. This approach has the disadvantage in that it can interfere with the training program of the athlete and, if the athlete is becoming over-trained, additional maximal exercise performance testing would only serve to worsen their condition. Furthermore, if the individual is very unfit, or has a serious medical condition such as cardiovascular disease, they simply may not have the capacity to undergo a maximal exercise performance test, and in some cases performance of such a test could prove life threatening for the individual.
Therefore, methods and devices for assessing the recovery state of an individual and/or predicting the individual's ability to perform an athletic activity at their optimal capacity are required. Such tests would be invaluable to an individual or athlete, including their coach and/or sport physiologist, for fitness or training purposes (including the design of appropriate training programs), and for achieving ultimate physical performance in their particular athletic pursuit. If such tests are also able to sensitively detect changes in autonomic balance that precede the presence of disease, then they will also be useful for predicting the future risk of cardiovascular disease and/or for monitoring disease progression in the wider (non-athletic) population.
Reference herein to a patent document or other matter which is given as prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country as at the priority date of any of the claims in this application.